The semiconductor sample considered here has the form of a disk. This can be a so-called wafer or a part thereof. Preferably it concerns a sample, which is coated with gallium nitride at its top side. The top side, however, may also comprise other semiconductor materials, like silicon, germanium, gallium arsenide, indium phosphide or other compositions containing elements of the 3rd and 5th period of the periodic table of the elements, i. e. so-called III-V-semiconductors.
Gallium nitride has lately attained a steadily increasing importance: With this material it is possible to fabricate opto-electronic devices, such as light emitting diodes (LEDs) and diode lasers with blue radiation emission, electronic high frequency elements, electronic elements to operate at very high operating temperatures and interesting micro-mechanical devices. In contrast to other III-V-semiconductors, e.g. gallium arsenide, gallium nitride is very stable against wet-chemical procedures usually used in semiconductor manufacturing. Photo-electrochemical etching is an important process step for the measurement of the doping profile of a semiconductor. Measurement of the doping profile uses the fact, that the interface between the electrolyte and the semiconductor surface forms a Schottky contact. The Schottky contact permits etching of the semiconductor, if a voltage is applied to the interface in forward direction, or if the interface is illuminated with light. The Schottky contact also permits to measure the carrier concentration (doping) of the semiconductor, if the capacity of the interface is measured at various voltages in reverse direction. Therefore, the doping profile of the semiconductor sample can be determined by alternating etching and measuring steps. Also in manufacturing steps for the production of gallium nitride devices photo-electrochemical etching can be an important process step.
A procedure and an equipment of the kind initially specified are known from the articles “Dry and Wet Etching for Group III-Nitrides” (I. Adesida, C. Youtsey, A. T. Ping, F. Khan, L. T. Romano, G. Bulman; MRS Internet J. Nitride Semiconductor Res. 4S1, G1.4 (1999), in particular FIG. 4) and “Smooth n-type GaN surfaces by photoenhanced wet etching” (C. Youtsey, I. Adesida; Applied Physics Letters, Vol. 72 (1998), p. 560–562). A semiconductor sample of negatively doped gallium nitride on a Teflon plate is positioned horizontally in a container. The container is filled with aqueous diluted caustic potash solution (KOH), and the semiconductor sample is illuminated with UV light from the top. As known from the electrochemistry of electrolyte semiconductor interfaces, the contact area between the semiconductor sample of negatively doped gallium nitride and the diluted caustic potash solution (KOH) form a Schottky contact. If this contact area is irradiated with light of sufficiently high energy, in the semiconductor sample directly underneath the contact area electron-hole-pairs are generated. This will lead to a photo-current, if an electric circuit between the electrolyte liquid and the semiconductor sample is closed. The electrons move from the contact area to the electric contact via the negatively doped semiconductor sample. The holes can release electrons of the semiconductor atoms at the semiconductor surface, and thus the semiconductor surface may be etched. This etching of the semiconductor material may be supervised by measuring the photo-current; it is called photo-electro-chemical etching of the semiconductor.
For the well-known equipment cited above it is to be regarded as unfavorable that always the entire semiconductor sample comes into contact with the electrolyte liquid, and that generally it is not possible to etch locally a limited range of the semiconductor sample for local measurements. If semiconductor materials, such as e. g. gallium nitride, are etched, gas bubbles develop at the contact area. These gas bubbles rise towards the incident UV light in an upward direction, what disturbs an even irradiation of the sample with light. Solid residues from the etching procedure may remain lying on the contact area. They also disturb the incident light irradiation and the further etching procedure. Also traces of gas bubbles may possibly be visible after the etching procedure on the surface structure of the semiconductor sample, because the gas bubbles increase slowly, before they separate from the surface and rise. So this procedure does not produce reproducibly clean and smooth surfaces, seen on a macroscopic scale.
From the book “Etching of III-V-Semiconductors” (P. H. L. Notten, J. E. A. M. van den Meerakker, J. J. Kelly; Elsevier Science Publishers Ltd. 1991, ISBN 0-946395-84-5, pages 43 to 46, in particular FIG. 3,4 on page 44) it is well-known, that it is an advantage to supply fresh electrolyte liquid to the contact area during the photo-electrochemical etching of a semiconductor sample. In this equipment the semiconductor sample is mounted horizontally on a plate. This plate simultaneously forms the bottom of the electrolyte container. The top of the electrolyte container consists of glass. The top contains a window, through which light can be conducted to the contact area using a light conductor. The top of the electrolyte container also contains an inlet for fresh electrolyte liquid and an outlet for the liquid. The inlet is implemented in such a way that electrolyte liquid flows over the contact area in a laminar flow without developing turbulences. This is meant to assure that during the entire etching procedure fresh electrolyte liquid may flow, without disturbing the light irradiation of the contact area by any turbulences of the liquid. In the description of FIG. 4 it is noted that a measuring instrument can be attached to the outlet to analyse the liquid after contact with the contact area.
This equipment has the disadvantage that the constant supply of electrolyte liquid leads to a relatively high consumption of electrolyte liquid. In addition this equipment is just an experimental device; it merely serves scientific investigations of the etching process. It has further disadvantages, which have been mentioned already with the first well-known procedure above, and is was not planned to be used with samples made of gallium nitride.
It would be desirable and advantageous to provide an improved method for etching of a semiconductor sample, in particular of gallium nitride, to obviate prior art shortcomings
It would also be desirable and advantageous to provide an improved device for etching of a semiconductor sample with better etching results, when etching over a longer time is involved.